The present invention generally relates to wireless transmitting devices, and more particularly, to techniques for linearization of a power amplifier.
In wireless communications systems, such as the Universal Mobile Telephone System (UMTS), a power amplifier is often driven into the non-linear operating region to increase transmission efficiency. Unfortunately, this causes spectral broadening and in-band distortion. As such, various techniques have been developed to linearize the operation of the transmitter (often referred to as linearization of the power amplifier) while still operating the power amplifier efficiently.
One such technique is predistortion. Generally speaking, in predistortion the signal-to-be-transmitted is first distorted in a complementary fashion to the distortion introduced by the power amplifier. In effect, the distortion introduced into the signal-to-be-transmitted cancels the distortion caused by the non-linear operation of the power amplifier. Thus, the overall impression is that the transmitter includes an ideal linear amplifier.
For example, in the non-linear operating region the power amplifier may produce higher order intermodulation products into the signal-to-be-transmitted. As such, to reduce, if not substantially eliminate these higher order intermodulation products, a predistortor generates and injects second order harmonic products into the signal-to-be transmitted as illustrated in FIG. 6. In particular, an intermediate frequency (IF) input signal 101 (IF 101) is applied to mixer 170, which, as known in the art, mixes IF 101 with a local oscillator (LO) signal 171 operating at the desired radio frequency (RF), to provide an upconverted RF signal to bandpass filter (BPF) 175. The LO signal 171 frequency value is equal to the desired RF frequency±the IF frequency, depending on the choice of the upper side or the lower side of the upconversion. For example, in the context of a wireless device operating in accordance with UMTS, the IF frequency is typically 380 MHz, the LO frequency is in the range of 1.54 Giga Hertz (GHz) (109 Hz) to 1.60 GHz. BPF 175 filters the upconverted RF signal to provide a filtered RF signal 176 to predistorter 100. In terms of this example, the RF signal frequencies are on the order of 1.92 GHz to 1.98 GHz. As such, predistorter 100 represents a microwave circuit as known in the art. Predistorter 100 includes coupler 105, input matching network 110, coupler 115, intermodulation generator (IM) 120, BPF 130, amplifier (AMP) 135, phase shifter 140, and amplitude adjuster 145. The filtered RF signal is applied to coupler 105. Coupler 105 is a “weak” directional, coupler and provides most of the filtered RF signal to coupler 115, via input matching network 110. However, a portion of the filtered RF signal (e.g., on the order of 20 dB (decibels)) is extracted by coupler 105 and provided to IM 120. The latter provides a non-linear function for generating second order harmonic products from the extracted portion of the filtered RF signal. The second order harmonic products provided by IM 120 are further shaped, via BPF 130, and then may be further amplified by amplifier (Amp) 135 to provide second order harmonic products of sufficient level to cancel intermodulation products. The phase and amplitude of the second order harmonic products from Amp 135 are further adjusted via phase shifter 140 and amplitude adjuster 145 for application to directional coupler 115. Coupler 115 injects the adjusted second order harmonic products 146 back into the filtered RF signal and provides a combined signal—the filtered RF signal and the adjusted second order harmonic products—to power amplifier 185, which provides RF output signal 186 (RF 186) for transmission. Digital signal processor (DSP) 190 controls phase shifter 140 and amplitude adjuster 145, via control signals 191 and 192, respectively, to adjust the phase and amplitude of the second order harmonic products such that amplification of the combined signal by power amplifier 185 now reduces, if not substantially eliminates, the higher order intermodulation products produced by the power amplifier. DSP 190 may include a look-up table (LUT) 195 in memory as known in the art for use in generating control signals 191 and 192, and digital-to-analog-converters (not shown) for generating the control signals.
Since IM 120 generates second order harmonic products, these signals operate at twice the RF signal frequency. As such, as wireless transmission frequencies continue to increase into the GHertz range, e.g., 2 GHz and higher, this in effect doubles the operating frequency requirements for the components of predistorter 100 of FIG. 6. Unfortunately, having to design at twice the operating frequency requirements for components, such as phase shifter 140, further increases their cost and affects the complexity of the overall predistorter implementation. For example, if the RF signal frequency is 2 GHz, designing at twice this operating frequency requires that the phase shifter operate at RF signal frequencies of 4 GHz. This imposes more stringent requirements on the permissible level of insertion loss, phase ripple and parasitics with respect to the component inductors, capacitors and varactors that typically comprise the phase shifter—all of which adds to the cost. In addition, more care must be taken in the circuit design and layout of the phase shifter to ensure manufacturability.